The present invention relates generally to improvements in temperature sensing devices of the type employing a thermistor, and more particularly the invention relates to a temperature sensing device which is well suited for use in detecting the temperature of the air blown from an evaporator or the temperature at the surface of the evaporator fins in an automobile air conditioning system, although the device is not limited in applications in any way.
Conventionally, with automobile air conditioning systems it has been the practice so that the temperature of the air blown from the evaporator or the temperature at the surface of the evaporator fins is detected by a thermistor whereby when the detected temperature drops below a predetermined temperature, the electromagnetic clutch is disengaged and the operation of the compressor is stopped thus preventing the formation of frost in the evaporator.
However, this kind of known air conditioning system is disadvantageous in that as the compressor is operated intermittently, the resulting controlling temperature difference for the temperature of the air blown from the evaporator becomes as high as over 8.degree. C. and hence the temperature of the air blown into the interior of the vehicle compartment is varied considerably thus deteriorating the feeling of air conditioning.
The inventors have conducted various experiments and studies on these problems and have discovered that in the known systems the thermistor element size is so large that the heat capacity of the entire temperature sensing device including the thermistor inevitably becomes large and the resulting thermal time constant indicative of a thermal response characteristic with respect to rapid temperature changes becomes about over 60 seconds, thus giving rise to the above-mentioned problems.
It has also been found that the heat from the parts external to the sensing section is transmitted to the thermistor by way of its external circuit connection lead wires thus further promoting the above-mentioned deficiencies.